Low-dimensional hyperthin fes2 nanostructures for electrocatalysis

ABSTRACT

Electrodes are provided comprising a FeS2 electrocatalytic material, the FeS2 electrocatalytic material comprising FeS2 nanostructures in the form of FeS2 wires, FeS2 discs, or both, wherein the FeS2 wires and the FeS2 discs are hyperthin having a thickness in the range of from about the thickness of a monolayer of FeS2 molecules to about 20 nm. The FeS2 nanostructures may be polycrystalline comprising a non-pyrite majority crystalline phase. The FeS2 nanostructures may be in the form of FeS2 discs wherein substantially all the FeS2 discs have at least partially curved edges.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 62/307,191 that was filed Mar. 11, 2016, the entirecontents of which are hereby incorporated by reference.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under NSF-DMR-1451326awarded by the National Science Foundation and under W911NF-14-1-0443awarded by the Army Research Office. The government has certain rightsin the invention.

BACKGROUND

A key green energy initiative is the discovery of efficient, stable, andelemental abundant electrocatalysts for the water splitting reactions,i.e. the hydrogen evolution reaction (HER) and the oxygen evolutionreaction (OER).¹⁻³ Water splitting with HER and OER electrocatalystsplay a vital role in converting solar energy into chemical energy viaartificial photosynthesis, and also provides a pathway to use water (asopposed to natural gas) as a feedstock for hydrogen production.⁴⁻⁶Nanostructured transition-metal chalcogenides have previously beenstudied as HER electrocatalysts.⁷⁻¹⁵ While other transition-metalchalcogenides have also been studied as HER catalysts (e.g. WS₂, NiS₂,COS₂, NiSe₂, and CoSe₂),^(8,16) there have been only limited reports onthe catalytic activity of FeS₂,^(8,17) and none have shown highefficiency for FeS₂.

SUMMARY

Provided are FeS₂ electrocatalytic materials, electrocatalytic systemscomprising the FeS₂ electrocatalytic materials, and related methods.

In one aspect, electrodes comprising FeS₂ electrocatalytic materials areprovided. In an embodiment, an electrode comprises a FeS₂electrocatalytic material, the FeS₂ electrocatalytic material comprisingFeS₂ nanostructures in the form of FeS₂ wires, FeS₂ discs, or both,wherein the FeS₂ wires and the FeS₂ discs are hyperthin having athickness in the range of from about the thickness of a monolayer ofFeS₂ molecules to about 20 nm. The FeS₂ nanostructures may bepolycrystalline comprising a non-pyrite majority crystalline phase. TheFeS₂ nanostructures may be in the form of the FeS₂ discs, whereinsubstantially all the FeS₂ discs have at least partially curved edges.

In another aspect, electrochemical systems for catalyzing anelectrochemical reaction are provided. In an embodiment, the systemcomprises an electrochemical cell configured to contain a fluidcomprising an electrochemical reactant; the electrode described above incontact with the fluid; a counter electrode in electrical communicationwith the electrode.

In another aspect, methods for making the electrode described above areprovided. In an embodiment, the method comprises injecting a firstprecursor solution comprising sulfur (S), the first precursor solutionhaving a first temperature, into a second precursor comprising iron(Fe), the second precursor solution having a second temperature, to forma reaction mixture, and allowing the reaction mixture to react at areaction temperature for a reaction time, wherein a ratio of Fe:S in thefirst and second precursor solutions is selected to achieve thenanostructures in the form of FeS₂ wires, FeS₂ discs, or both, whereinthe FeS₂ wires and the FeS₂ discs are hyperthin having the thickness inthe range of from about the thickness of a monolayer of FeS₂ moleculesto about 20 nm. The FeS₂ wires and the FeS₂ discs may be polycrystallinecomprising a non-pyrite majority crystalline phase.

In another aspect, methods of using the electrode described above tocatalyze an electrochemical reaction are provided. In an embodiment, themethod comprises exposing the FeS₂ electrocatalytic material of theelectrode described above to a fluid comprising an electrochemicalreactant under conditions sufficient to induce the reduction of theelectrochemical reactant at the FeS₂ electrocatalytic material-fluidinterface to form a reduction product or under conditions sufficient toinduce the oxidation of the electrochemical reactant at the FeS₂electrocatalytic material-fluid interface to form an oxidation product.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings, wherein like numeralsdenote like elements.

FIGS. 1A-1C show a schematic representation and FIGS. 1E-1G show TEMimages of the 1D and 2D FeS₂ structure formation. FIGS. 1A and 1E arethe Fe⁰ nanoparticles that are formed in the absence of sulfur. FIGS. 1Band 1F are the FeS₂ wires formed from 1:6 Fe:S precursor solution. FIGS.1C and 1G are the FeS₂ discs formed in the 1:24 Fe:S precursor. FIG. 1Dis the schematic of ˜2.7 nm wide ligand interstitial layer thatseparates both wires and discs to form their respective bulk structures.

FIGS. 2A-2C show the time dependent TEM characterization for the wiresat 0.5, 5, and 240 minutes, respectively. FIGS. 2E-2G shows the timedependent TEM characterization for the discs at 0.5, 5, and 240 minutes,respectively. FIGS. 2D, 2H show SEM characterization of the final wireand disc formations cast on Si substrates, respectively. FIG. 2I showstime dependent EDS measurements with the inset focusing on the first 15minutes of the reaction. FIG. 2J shows the plot of the Ramanspectroscopy data to establish phase identification.

FIGS. 3A-3B show the electrochemical characterization of FeS₂ discs,wires, and cubes in 0.1 M pH 7 phosphate buffer solution (PBS) for thehydrogen evolution reaction. FIG. 3A shows experimental linear sweepvoltammograms at 1 mV/s (solid lines) for the champion FeS₂ discs,wires, and cubes coated on glassy carbon along with a bare Pt electrodeand a bare glassy carbon electrode. Also shown are the correspondingbest-fit single-electron Butler-Volmer equations (dashed lines) for eachelectrode. FIG. 3B shows a Tafel plot showing the experimental data inthe Tafel region (circle markers) with the corresponding Tafel slopes(solid lines) for Pt, FeS₂ discs, wires, and cubes.

FIGS. 4A-4D show hydrogen evolution electrochemical reactivity mapsobtained via scanning electrochemical microscopy (SECM). FIG. 4A shows aschematic of the SECM experiment showing hydrogen collection on the SECMtip electrode. FIG. 4B shows the reactivity map for bare glassy carbonelectrode. FIGS. 4C and 4D show electrochemical reactivity maps for theHER on Pt and FeS₂ discs on glassy carbon, respectively.

FIG. 5 shows the growth of the peak at 700 nm from UV-Vis-IRmeasurements of FeS₂ wire and disc formations as tracked over the courseof the reaction with the inset focusing on the first 30 minutes toillustrate the relative changes.

FIGS. 6A-6B show TEM images of crystalline FeS₂ discs. A FeS₂ disc with1 m diameter is shown in FIG. 6A, the circle indicates the area whereSAED pattern (insert) was taken. FIG. 6B shows a high resolution TEMimage of the FeS₂ disc observed from its [001]direction, FFT of thesquared area (insert) shows diffraction peaks from (020), (110) and(200) planes.

FIG. 7 shows a TEM image of the 3D cubes for comparison to wire and discstructures.

FIG. 8A shows Linear Sweep Voltammograms (LSVs) of Pt and FeS₂ Discs ata scan rate of 1 mV/s showing the mass-transfer limited regime. FIG. 8Bshows LSVs in FIG. 8A overlaid with a LSV of Pt at a scan rate of 50mV/s. FIG. 8C shows the LSVs in FIG. 8B with the current densitynormalized by scan rate.

FIG. 9 shows current vs. Time plots for Pt and FeS₂ discs in stirred 0.1M pH 7 phosphate buffer held at a constant potential of −0.14 V vs. RHE.

FIG. 10 shows electrochemical surface area normalized linear sweepvoltammograms of Pt, and FeS₂ discs, wires, and cubes in 0.1 M pH 7phosphate buffer solution (PBS) for the hydrogen evolution reaction at 1mV/s.

FIG. 11A shows a TEM image of a circular FeS₂ 2D disc having an entirelycurved edge. FIG. 11B shows a TEM image of an elliptical FeS₂ 2D dischaving an entirely curved edge. FIG. 11C shows a SEM image includingelliptical FeS₂ 2D discs in which a portion of each edge is straight.

FIG. 12 shows a schematic of an illustrative electrochemical systemincluding a working electrode based on a FeS₂ electrocatalytic material.The electrochemical system may be used to catalyze the hydrogenevolution reaction.

DETAILED DESCRIPTION

Provided are FeS₂ electrocatalytic materials, electrocatalytic systemscomprising the FeS₂ electrocatalytic materials, and related methods. Theinvention is based, at least in part, on the discovery of certain FeS₂electrocatalytic materials (and methods for making the materials) whichexhibit greatly superior catalytic performance as compared toearth-abundant FeS₂. Moreover, the catalytic performance is comparableto platinum in neutral pH conditions and the catalytic activity ismaintained over long periods of time (e.g., hundreds of hours), therebyfacilitating the practical use of the FeS₂ electrocatalytic materialsfor catalyzing certain electrochemical reactions, including the hydrogenevolution reaction (HER), for the first time.

The FeS₂ electrocatalytic materials comprise nanostructured FeS₂.“Nanostructured FeS₂” refers to a solid material of FeS₂ in the form ofdistinct, distinguishable nanostructures (e.g., as visualized viatransmission electron microscope (TEM) images) having at least onedimension of about 1000 nm or less or about 100 nm or less. Thenanostructures may be characterized by their shape and dimensions. Insome embodiments, the nanostructures are wires characterized by a lengthl_(w), width w_(w), and thickness t_(w), wherein l_(w)>>w_(w), t_(w).The width w_(w), and thickness t_(w) may be of similar magnitudes. Whenw_(w)˜t_(w), the wires may be characterized by a length l_(w) and adiameter d_(w), although the cross-sectional shape of the wires may notbe perfectly circular.

In other embodiments, the nanostructures are discs characterized by alength l_(d), width w_(d), and thickness t_(d), wherein l_(d),w_(d)>>t_(d). The length l_(d) and width w_(d) and may be of similarmagnitudes. The discs may be characterized by the curvature of its outerperimeter, i.e., its edge. The discs may have at least a partiallycurved edge. This means that at least a portion of the edge is curved,although other portions of the edge may be non-curved, i.e.,characterized by a straight line. In embodiments, the entire edge iscurved along its length. The degree of curvature may be the same ordifferent along the length of the edge.

The discs may also be characterized by the overall shape defined bytheir edges. The discs may be characterized as being circular orelliptical. The terms are not meant to imply that the shapes areperfectly regular. For example, in some embodiments, a disc may have aportion of its edge which is straight and the remaining portion curved.Moreover, the degree of curvature along the curved portion may vary.However, these terms do distinguish discs which have all straight edgesand generally sharp corners, e.g., hexagonally shaped discs.

FIG. 11A shows a TEM image in which a circular disc is labeled. Inembodiments, circular discs may be characterized by a diameter d_(d). Insuch cases d_(d)>>t_(d). As shown in the figure, the circular disc hasl_(d)=245 nm and w_(d)=238 nm. Since l_(d)˜w_(d), the plane of the disccan be defined by a circle having a diameter d_(d) of about 240 nm. Theentire edge of this circular disc is curved. FIG. 11B shows a TEM imagein which an elliptical disc is labeled. In embodiments, elliptical discsmay be characterized by a major diameter d_(d, major) and a minordiameter d_(d, minor). In such cases d_(d, major) andd_(d, minor)>>t_(d). As shown in the figure, the elliptical disc hasd_(d, major)=348 nm and d_(d, minor)=143 nm. The entire edge of thisdisc is curved. FIG. 11C shows a SEM image of a plurality of FeS₂ 2Ddiscs. Three discs are outlined. These discs each have a portion of theedge which is straight (indicated by the arrows) with the remainder ofthe edge curved. These discs may still be characterized as beingelliptical.

In embodiments, substantially all the nanostructures of the FeS₂electrocatalytic material are in the form of the discs. By“substantially” it is meant that the nanostructures may include a smallamount of a different type of nanostructure, but that the different typeof nanostructure does not materially affect the catalytic properties ofthe FeS₂ electrocatalytic material. In other words, the catalyticproperties of the FeS₂ electrocatalytic material are controlled by thediscs.

In embodiments, substantially all the discs have at least a partiallycurved edge. In embodiments, at least about 25% of the discs have edgeswhich are entirely curved. This includes embodiments in which the amountis at least about 50% or at least about 75%. In embodiments,substantially all the discs have entirely curved edges. In embodiments,substantially all the discs are either circular or elliptical. In eachcase, the term “substantially” has a meaning analogous to that describedabove.

Without wishing to be bound to any particular theory, it is thought thatthe presence of curved edges is related to the catalytic efficiency ofthe FeS₂ nanostructures. Curved edges may provide catalytically activeedge sites which allow optimal adsorption of key reaction intermediatesdue to the presence of stretched and/or dangling bonds. Suchstretched/dangling bonds would not be as concentrated at straight edgeswhere FeS₂ molecules would be better able to orient into a bondingconfiguration.

The nanostructures are “hyperthin,” by which it is meant thenanostructures have at least one dimension in the range of from aboutthe thickness of a monolayer of FeS₂ molecules to about 20 nm. Thisincludes embodiments in which the nanostructures have at least onedimension which is at least about the thickness of a monolayer of FeS₂molecules, a bilayer of FeS₂ molecules, or a trilayer of FeS₂ moleculesbut less than about 20 nm, about 15 nm, about 10 nm, or about 5 nm. Thethickness may be that of a monolayer of FeS₂ molecules, a bilayer ofFeS₂ molecules, or a trilayer of FeS₂ molecules. The thickness may be inthe range of from about 0.1 nm to about 20 nm, from about 0.1 nm toabout 10 nm, from about 0.1 nm to about 5 nm, or from about 0.1 to about1 nm.

In some embodiments, the nanostructures are hyperthin wires in whicht_(w) (or d_(w)) is the hyperthin dimension having the values asdescribed in the paragraph immediately above. The term “1D” (onedimensional) may be used to describe such nanostructures. The lengthl_(w) of the hyperthin wires is not particularly limited. In someembodiments, the length is at least about 50 nm. This includesembodiments in which the length is at least about 100 nm, about 500 nm,about 1 μm, about 5 μm, about 10 μm, about 100 μm or in the range offrom about 50 nm to about 100 μm. FIG. 1B shows a schematic illustrationof a hyperthin 1D FeS₂ wire.

In other embodiments, the nanostructures are hyperthin discs in whicht_(d) is the hyperthin dimension having the values as described withrespect to the hyperthin wires. The term “2D” (two dimensional) may beused to describe such nanostructures. The other dimensions of thehyperthin discs (e.g., l_(d), w_(d) (or d_(d)) are not particularlylimited. In some embodiments, these dimensions may assume a value of atleast about 50 nm. This includes embodiments in which the dimensions areat least about 100 nm, about 500 nm, about 1 μm, about 5 μm, about 10μm, or in the range of from about 50 nm to about 10 m. FIG. 1C shows aschematic illustration of a hyperthin 2D FeS₂ disc.

Hyperthin nanostructures may also be identified by their transparency astheir extreme thinness renders them significantly more transparent ascompared to thicker nanostructures (e.g., compare the TEM images showingtransparent hyperthin 1D FeS₂ wires in FIG. 2C and transparent hyperthin2D FeS₂ discs in FIG. 2G to opaque cubes in FIG. 7). In someembodiments, the hyperthin nanostructures exhibit a transparency in therange of from about 40% to about 60% at about 200 keV or of about 50% atabout 200 keV.

The nanostructures do not include nanoparticles, nanocubes (FIG. 7), orsimilar nanostructures in which all three dimensions of thenanostructures are of similar magnitude. The term “3D”(three-dimensional) is used in the Examples to describe suchnanostructures.

FeS₂ electrocatalytic materials comprising combinations ofnanostructures having different shapes may be used. The numeric valuesfor the dimensions described above may refer to the average value of acollection of nanostructures.

FeS₂ electrocatalytic materials may be characterized by the phase of thematerial. In embodiments, the FeS₂ electrocatalytic material ispolycrystalline, i.e., comprising more than one crystalline phase (asopposed to single crystalline composed of a single crystalline phase).Polycrystalline can also refer to FeS₂ electrocatalytic materialscomprising regions of amorphous phase (lacking any crystalline order) inaddition to one or more crystalline phases. The FeS₂ electrocatalyticmaterials may be further characterized by the identity of the majorityphase in the material, i.e., the phase which is present at the greatestamount. In embodiments, the FeS₂ electrocatalytic material ispolycrystalline comprising non-pyrite majority crystalline phase. Thismeans that the crystalline phase present in the greatest amount is notpyrite phase. In embodiments, the FeS₂ electrocatalytic material ispolycrystalline comprising marcasite as the majority crystalline phase.This includes embodiments in which there is greater than 50% marcasitephase, greater than 60%, greater than 70%, or greater than 80%. In suchembodiments, other crystalline phases which may be present at smalleramounts include pyrite phase and pyhrrotite phase. As noted above,amorphous phase may also be present. The phase of the FeS₂electrocatalytic materials can be evaluated using High ResolutionTransmission Electron Microscopy and electron diffraction as describedin the Example, below.

The FeS₂ electrocatalytic materials are distinguished from conventionalFeS₂ materials which are highly crystalline, i.e., substantiallysingle-crystalline, and FeS₂ materials which may be polycrystalline butcomprise pyrite phase as the majority crystalline phase. On this basis,the FeS₂ electrocatalytic materials are distinguished from the materialsdescribed in the following references: Macpherson, H. A., et al.,ACSNano 2012, 6, 8940-8949; Bai, Y. X., et al., J. Phys. Chem. C 2013,117, 2567-2573; Kirkeminde, A., et al., ACS Appl. Mater. Interfaces2012, 4, 1174-1177; Kirkeminde, A., et al., Nanotechnology 2014, 25,205603; Ennaoui, A., et al., J. Electrochem. Soc. 1992, 139, 2514-2518;and Gong, M., et al., Sci. Rep. 2013, 3, 2092. Despite the lack of phasepurity of the present FeS₂ electrocatalytic materials as compared to theconventional FeS₂ materials, it has been found that the presentmaterials have extremely high catalytic efficiencies, e.g., for thehydrogen evolution reaction.

Individual nanostructures may assemble together in the presence of aligand to form a larger structure (e.g., a “microstructure”) in whichthe individual nanostructures are held together via ligand-ligandinteractions. By way of illustration, a plurality of hyperthin 1D FeS₂wires may assemble to form a bundle of hyperthin 1D FeS₂ wires, whereinneighboring wires are substantially aligned (i.e., aligned, but notnecessarily perfectly aligned) along their lengths and separated by aligand layer. (See, e.g., FIGS. 1D and 1F.) As shown schematically inFIG. 1D, the “heads” of ligand molecules bind to the surface of the 1DFeS₂ wires, thereby decorating the length of the 1D FeS₂ wires. The“tails” of the ligand molecules bind to each other, thereby associatingneighboring 1D FeS₂ wires. The spacing between neighboring wires due tothe ligand can be about the length of the ligand, e.g., less than about5 nm, less than about 4 nm, in the range of from about 2 nm to about 3nm. The bundles themselves are generally distinct, identifiablestructures, although the overall shape and dimensions of individualbundles may vary. (See, e.g., FIGS. 2C and 2D.) The bundles may berandomly oriented with respect to one another to define a plurality ofirregularly shaped pores in the FeS₂ electrocatalytic material. (See,e.g., FIG. 2D.)

Similarly, a plurality of hyperthin 2D FeS₂ discs may assemble to form astack of hyperthin 2D FeS₂ discs, wherein neighboring discs aresubstantially aligned (i.e., aligned, but not necessarily perfectlyaligned) along their planes and separated by a ligand layer. The spacingbetween neighboring discs may be as described above with respect to thebundles of wires. The stacks themselves are generally distinct,identifiable structures, although the overall shape and dimensions ofindividual stacks may vary. (See, e.g., FIG. 2H.) The stacks may berandomly oriented with respect to one another to define a plurality ofirregularly shaped pores in the FeS₂ electrocatalytic material. (See,e.g., FIG. 2H.)

The ligand which facilitates the formation of the larger structures maybe that which is used in making the nanostructured FeS₂, as furtherdescribed below.

The nanostructured FeS₂, including nanostructured FeS₂ assembled vialigand-ligand interactions to form microstructures, may be used in theform of a film, layer, or coating on an underlying substrate to providean electrode. A variety of substrates may be used. The substrate istypically a conductive substrate. By way of illustration, carbonsubstrates (e.g., glassy carbon) may be used.

In addition to the ligand described above, the FeS₂ electrocatalyticmaterial may comprise other components, e.g., binders, fillers, etc. Theparticular components and amount of components may depend upon theelectrochemical reaction to be catalyzed. For catalyzing the HER, thebinder may be an ionic conductor, e.g., an ionomer. Perfluorinatedionomers may be used, e.g., Nafion. The FeS₂ electrocatalytic materialmay comprise such binders in an amount from about 0.01% to about 25% byweight of the material, from about 0.5% to about 15% by weight of thematerial, or from about 10% to about 15% by weight of the material. Forthe HER, the filler may be conductive carbon, e.g., carbon black such asacetylene black. The FeS₂ electrocatalytic material may comprise suchfillers in an amount from about 1% to about 50% by weight of thematerial, from about 5% to about 25% by weight of the material, or fromabout 5% to about 15% by weight of the material.

The FeS₂ electrocatalytic material may be used to catalyze a variety ofelectrochemical reactions. In one embodiment, the FeS₂ electrocatalyticmaterial may be used to catalyze the hydrogen evolution reaction (HER),an electrochemical reaction in which hydrogen (H₂) is produced via theelectrolysis of water (H₂O). The hydrogen evolution reaction may becarried out in an electrochemical cell comprising an anode and a cathodein contact with an electrolyte solution (e.g., a solution of water and awater-soluble electrolyte such as H₂PO₄). The hydrogen evolutionreaction may be conducted at about neutral pH (pH ˜7). Application of anelectrical potential across the anode and the cathode causes dissociatedhydrogen ions (H⁺) to migrate to the cathode where they are reduced byfree electrons to produce H₂. The hydrogen gas may desorb from thecathode and be collected from the electrochemical cell. At the anode,water reacts to form oxygen (O₂), hydrogen ions and electrons. The FeS₂electrocatalytic material may be used to catalyze the reduction reactionof HER, i.e., the reduction of hydrogen ions to H₂ at the cathode. TheFeS₂ electrocatalytic material may also be used to catalyze the directreduction of water, i.e., 2H₂O+2e⁻->H₂+2OH⁻.

In another embodiment, the FeS₂ electrocatalytic material may be used tocatalyze the reduction of carbon dioxide (CO₂), an electrochemicalreaction in which formate, carbon monoxide (CO), other oxygenated carboncompounds, or hydrocarbons is produced. The CO₂ reduction may be carriedout in an electrochemical cell comprising an anode and a cathode incontact with an electrolyte solution (e.g., a solution of water and awater-soluble electrolyte such as Na₂SO₄; an ionic liquid such as1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; or anorganic solvent such as acetonitrile and an organic-soluble electrolytesuch as tetrabutylammonium hexafluorophosphate). The CO₂ reduction maybe conducted at a pH in the range of 4 to 8. Application of anelectrical potential across the anode and the cathode causes dissolvedCO₂ molecules to migrate to the cathode where they are reduced by freeelectrons to produce the radical, CO₂.⁻, which will react further toform the final product(s). The liquid or gas products may be collectedfrom the electrochemical cell. In a solution of water, water reacts toform O₂, hydrogen ions and electrons. The FeS₂ electrocatalytic materialmay be used to catalyze the reduction of CO₂, i.e., the reduction of CO₂at the cathode.

Other electrochemical reactions which may be catalyzed by the FeS₂electrocatalytic materials include oxidation reactions. In oneembodiment, the FeS₂ electrocatalytic material may be used to catalyzethe oxidation of H₂ to produce protons and free electrons at the anodeof a hydrogen fuel cell. The protons are drawn through an electrolytesolution in contact with the anode to the cathode. The free electronsare also drawn from the anode to the cathode through an external circuitto produce electricity. At the cathode, an oxidizing agent such as O₂reacts with the protons and some of the free electrons to produce water.

The FeS₂ electrocatalytic material may be characterized by itsefficiency at catalyzing a particular electrochemical reaction, e.g.,HER. In some embodiments, the efficiency is provided as theoverpotential at 0.1 mA/cm² as determined in a 0.1 M phosphate buffer ata pH of 7 and a scan rate of 1 mV/s. In such embodiments, the FeS₂electrocatalytic material may be characterized by an efficiency(overpotential) of no more than about 90 mV, no more than about 80 mV,or no more than 70 mV.

The FeS₂ electrocatalytic material may be characterized by its stabilityin catalyzing a particular electrochemical reaction over a period oftime. In some embodiments, the FeS₂ electrocatalytic material ischaracterized by the ability to generate H₂ by catalyzing the hydrogenevolution reaction, wherein the amount of generated H₂ remainssubstantially the same (e.g., within about +5%, about +10%, about +15%of the amount generated at an initial time point) for up to about 24hours, 50 hours, 100 hours, 125 hours, 150 hours, 200 hours, etc. Evengreater stabilities may be achieved, e.g., on the order of years. Thesevalues may refer to the stability at a neutral pH when measured asdescribed in the Example, below.

As demonstrated in the Example, below, it has been found that hyperthin2D FeS₂ discs are particularly efficient at catalyzing the reductionreaction of HER to generate H₂. Notably, the increased efficiency of thehyperthin 2D FeS₂ discs as compared to hyperthin 1D FeS₂ wires isgreater than would be expected based on the increased surface areaprovided by the hyperthin 2D FeS₂ discs. In particular, the ratio of theelectrochemical surface area for discs/wires was determined to be about2, while the ratio of the exchange current density for discs/wires wasdetermined to be between 5 to 7 (see Example).

Electrocatalytic systems comprising the FeS₂ electrocatalytic materialsare also provided. The electrochemical system may comprise anelectrochemical cell configured to contain a fluid comprising anelectrochemical reactant (e.g., a species to be reduced to form areduction product or a species to be oxidized to form an oxidationproduct); a working electrode comprising a FeS₂ electrocatalyticmaterial in contact with the fluid; and a counter electrode. Any of theFeS₂ electrocatalytic materials described herein may be used. Theselection of fluid depends upon the particular electrochemical reactionto be catalyzed. For the hydrogen evolution reaction, the fluid may bean electrolyte solution (e.g., a solution of water and a water-solubleelectrolyte), the electrochemical reactant may comprise hydrogen ionsand the reduction product may comprise H₂. Various materials for thecounter electrode may be used (e.g., Pt wire). The working electrode andthe counter electrode may be immersed in the fluid. The counterelectrode may be in electrical communication with the working electrode.

The electrocatalytic system may further comprise a power source inelectrical communication with the working electrode and the counterelectrode, the power source configured to apply an electrical potentialacross the working electrode and the counter electrode in order togenerate free electrons for electrochemically inducing the reductionreaction at the working electrode. Other components may be used in theelectrocatalytic system, e.g., a membrane separating the electrodes, acollection cell configured to collect the reduction product from theelectrochemical cell, etc.

The electrocatalytic system may be configured as a hydrogen fuel cell,in which case the fluid may be H₂, the electrochemical reactant maycomprise the H₂ and the oxidation product may comprise hydrogen ions andfree electrons.

An illustrative electrocatalytic system 1200 which may be used to carryout the hydrogen evolution reaction is shown in FIG. 12. Theelectrochemical system may comprise an electrochemical cell 1202configured to contain an aqueous electrolyte solution 1204; a workingelectrode 1206 comprising a FeS₂ electrocatalytic material 1208 incontact with the solution 1204; and a counter electrode 1210. A powersource 1212 is configured to apply an electrical potential across theworking electrode 1206 and the counter electrode 1210 in order togenerate free electrons for electrochemically inducing the reductionreaction at the working electrode 1206.

Methods of making the FeS₂ electrocatalytic materials are also provided.In one embodiment, the method may comprise injecting a first precursorsolution comprising sulfur, the first precursor solution having a firsttemperature, into a second precursor comprising iron, the secondprecursor solution having a second temperature, to form a reactionmixture and allowing the reaction mixture to react at a reactiontemperature for a reaction time to provide a FeS₂ electrocatalyticmaterial comprising nanostructured FeS₂. It has been found that thereaction temperature and the ratio of Fe:S in the first and secondprecursor solutions may be selected to achieve hyperthin FeS₂nanostructures having at least one hyperthin dimension having the valuesas described above and having a selected shape. Of these, the Fe:S, theratio has been found to be particularly critical to achieving hyperthin1D FeS₂ wires, hyperthin 2D FeS₂ discs, or a combination thereof.Moreover, it has been found that a much higher amount of sulfur isrequired to obtain these structures as compared to the stoichiometry ofthe final structures (i.e., 1:2 Fe:S). By way of illustration, a Fe:Sratio in the range of from about 1:4 to about 1:12 may be used toachieve hyperthin 1D FeS₂ wires (including from about 1:6 to about 1:10or about 1:8); a Fe:S ratio in the range of from about 1:24 to about1:38 may be used to achieve hyperthin 2D FeS₂ discs (including fromabout 1:28 to about 1:36 or from about 1:30 to about 1:34 or about1:32); and a Fe:S ratio in the range of from about 1:12 to about 1:20may be used to achieve a combination of hyperthin 1D FeS₂ wires andhyperthin 2D FeS₂ discs (including from about 1:14 to about 1:18 orabout 1:16). These ratios are molar (atomic) ratios.

Regarding the reaction temperature, the reaction temperature may be inthe range of from about 90° C. to about 160° C., from about 100° C. toabout 150° C., or from about 110° C. to about 130° C. The reaction timemay be selected to achieve the desired 1:2 stoichiometry. The reactionmay take place under inert conditions (e.g., in a degassed and inert gasfilled flask).

The first precursor solution comprises sulfur (e.g., elemental sulfur)and a first solvent. A variety of first solvents may be used (e.g.,phenyl ether). The first temperature of the first precursor solution mayassume different values, but is generally greater than room temperature.The first precursor solution may be made by mixing sulfur and the firstsolvent under heat and inert conditions for a period of time.

The second precursor solution comprises iron, a ligand and a secondsolvent. The iron may be derived from a decomposed iron precursor (e.g.,an iron salt such as FeI₂) in the second precursor solution. A varietyof ligands may be used, provided they are capable of acting as areducing agent for Fe²⁺ and as a capping layer on nanostructured FeS₂.Suitable ligands include alkylamines. The alkyl groups having variousnumbers of carbon atoms may be used, e.g., from 2 to 30, from 6 to 30,from 10 to 30, etc. Octadecylamine (ODA) is a suitable ligand. A varietyof second solvents may be used (e.g., phenyl ether). The secondprecursor solution may be made by injecting an iron precursor solutioncomprising the iron precursor and the second solvent into the ligand andmixing the second precursor solution under heat and inert conditions fora period of time sufficient to induce decomposition of the ironprecursor. The second temperature of the second precursor solution mayassume different values, but is generally greater than room temperatureand may be about the reaction temperature. The iron precursor solutionmay be made by mixing the iron precursor in a third solvent (e.g.,phenyl ether) under inert conditions for a period of time.

The Example below provides additional details (e.g., illustrativesuitable conditions, etc.) regarding the method of making the FeS₂electrocatalytic materials.

Methods of using the FeS₂ electrocatalytic material to catalyze anelectrochemical reaction are also provided. In one embodiment, themethod may comprise exposing a FeS₂ electrocatalytic material to a fluidcomprising an electrochemical reactant. If the electrochemical reactantis a species to be reduced (e.g., hydrogen ions), the exposure may occurin the presence of free electrons. The free electrons induce thereduction of the electrochemical reactant at the FeS₂ electrocatalyticmaterial-fluid interface to form a reduction product (e.g., H₂), whichmay be separated from the fluid and collected. The free electrons may bederived from an external power source in electrical communication withthe FeS₂ electrocatalytic material. If the electrochemical reactant is aspecies to be oxidized (e.g., H₂), the exposure results in theproduction of an oxidation product (e.g., hydrogen ions and freeelectrons), the free electrons which may be collected via an externalcircuit in electrical communication with the FeS₂ electrocatalyticmaterial. The conditions under which the FeS2 electrocatalytic materialis exposed in such methods depends upon the nature of theelectrochemical reaction. However, these conditions will be known. Byway of illustration, the conditions for achieving the reduction of waterto produce hydrogen gas via the hydrogen evolution reaction aredescribed in the Example below.

Example

In this Example, a scalable, solution-processing method for synthesizinglow-dimensional hyperthin FeS₂ nanostructures is provided. It is alsoshown that 2D FeS₂ disc nanostructures are an efficient and stablehydrogen evolution electrocatalyst. By changing the Fe:S ratio in theprecursor solution, it was possible to preferentially synthesize either1D wire or 2D disc nanostructures. The 2D FeS₂ disc structure has thehighest electrocatalytic activity for the hydrogen evolution reaction,comparable to platinum in neutral pH conditions. The ability of the FeS₂nanostructures to generate hydrogen was confirmed by scanningelectrochemical microscopy, and the 2D disc nanostructures were able togenerate hydrogen for over 125 hours.

Material preparation. FeI₂ (Sigma-Aldrich, anhydrous, ≥99.99%), sulfurpowder (Sigma-Aldrich, Colloidal), carbon black (Alfa-Aesar, acetylene,100% compressed, ≥99.9%), monobasic dihydrate sodium phosphate (AcrosOrganics, >99%), dibasic sodium phosphate (Acros Organics, anhydrous,ACS Reagent), (dimethylaminomethyl)ferrocene (Alfa Aesar, 98+%),octadecylamine (Acros Organics, technical grade, 90%), diphenyl ether(Acros Organics, 99%), chloroform (BDH, Anhydrous), methanol (FischerChemical, Certified ACS), tetrachloroethylene (Sigma-Aldrich, ACS) wereall used as received.

Wire and Disc Synthesis:

To make the FeS₂ wires, 0.5 mmol of FeI₂ and 1 mL of phenyl ether wasadded to a septa sealed vial in a N₂ flushed glovebox. This mixture wassonicated to form a uniform slurry, approximately 1 hour. In a roundbottom flask 12 g of ODA was added and degassed and backfilled withargon. The flask was then heated to 120° C., degassed and backfilledwith argon again, and allowed to cool to ˜80° C. The FeI₂ precursorsolution was injected into the flask containing ODA and heated back to120° C. and then left to stir for 1 hour to allow the precursor todecompose. In a separate flask, 128 mg of sulfur and 5 mL of phenylether was added and then degassed and backfilled with argon. This flaskwas heated to 70° C. and left to stir for 1 hour. After 1 hour, thesulfur solution was rapidly injected into the Fe-ODA solution and leftto react at 120° C. for 4 hours. The solution was allowed to cool to˜100° C. before injection of 10 mL of chloroform to prevent the solutionfrom congealing and then was transferred to centrifuge tubes, topped offwith methanol, and centrifuged at 4000 rpm for 7 minutes. Thesupernatant was poured off and an additional 5 mL of chloroform and 40mL of methanol was added, the solution mixed and centrifuged again. Thisstep was repeated two more times resulting in a fluffy black solid thatwas suspended in chloroform and stored under nitrogen. The sameprocedure was used to make discs except only 0.125 mmol of FeI₂ wasused. The average yield of the 2 syntheses was roughly around 70%,without the consideration of the mass loss during the cleaning process.

Materials Characterization.

UV-Vis absorbance spectra were taken on a UV-3600 Shimadzu UV-Vis-NIRspectrophotometer. X-ray powder diffraction was done at room temperatureusing monochromatic Cu-Kα radiation on a Bruker proteum diffractionsystem equipped with Helios multilayer optics, and APEX II CCD detectorand a Bruker MicroStar microfocus rotation anode X-ray source operatingat 45 kV and 60 mA. Powders were suspended in Paratone N oil and placedinto a nylon loop and mounted on a goniometer head. Transmissionelectron microscope (TEM) images were obtained using a field emissionFEI Tecnai F20 Xt. Energy dispersive X-ray (EDS) was done using an EDAXEDS with SiLi detector. Scanning electron microscope (SEM) images wereobtained using a LEO 1550 field emission SEM. Fourier transform infraredspectroscopy (FTIR) was performed using a Nicolet 6700. Ramanspectroscopy was performed using a Witec alpha 300 with a 633 nmwavelength laser.

Electrode Fabrication:

A suspension was made by combining 5 mg carbon black, 200 μL of 5 wt %Nafion solution (Fuel Cell Earth), and 1 mL of ˜50 mg/mL of either theFeS₂ wires or discs suspended in chloroform. Because of the differencein conductivity and particle size between the wires/discs and the cubes,the FeS₂ cubes suspension was fabricated by combining 5 mg carbon black,100 μL of 5 wt % Nafion solution, and 500 μL of FeS₂ cubes suspended inchloroform. Each nanostructure suspension was sonicated for ˜15 min then<10 μL was dropcast on a 3 mm diameter glassy carbon electrode (CHInstruments) and allowed to dry for ˜15 min before testing. The FeS₂cubes were synthesized using a previously reported method.²²

SECM Tip Electrode Fabrication:

A laser capillary pipet puller (Model P-2000, Sutter Instruments, USA),quartz capillaries (1 mm O.D., 0.3 mm I.D., 7.5 cm in length, SutterInstruments, USA), 200 μm diameter Pt wire (Electron Microscopy Sciences99.95% Pt wire), conductive silver epoxy (Circuit Works, USA), andsilver connection wire (30 AWG, Belden, USA) were utilized in thefabrication of 200 μm SECM tip electrodes. MicroCloth polishing disks(Buehler, Canada), alumina micropolish (1 μm, 0.3 μm, Buehler, Canada),and MicroCut 1200 grit silicon carbide grinding paper (P2500, Buehler,Canada) were utilized to polish SECM tips before experiments.

Microdisk Pt electrodes 200 μm in diameter were fabricated for the SECMtip. The 200 μm Pt wire was centered in the quartz capillary beforesealing the capillary to the wire and pulling to a tip with a lasercapillary pipet puller (Sutter P-2000). Course polishing of theelectrode tip with 1200 grit silicon carbide grinding paper wasperformed before fine polishing with 1 μm and 0.3 μm aluminamicropolish, consecutively. Silver connection wire lightly coated withsilver epoxy was inserted into the open end of the capillary tipelectrode such that the silver epoxy was connecting the silverconnection wire and the Pt wire. The SECM tip electrode was allowed todry in a Model 30GC Lab Oven (Quincy Lab Inc) at ˜100° C. for ˜20 minbefore using.

SECM Instrumentation:

All reactivity maps were performed in a custom-built SECM including thefollowing components from Newport: Vision Isostation air table(VIS2436-IG2-125A), faraday cage for air table, XPS MotionController/Driver with XPS-DRVP1 driver boards, 3-axis motion stage(VP-25XL-XYZL), 2 tilt stage motors (LTA-HS), and a Series 37 tiltstage. The SECM components were operated from an iMac computer viacustom designed LabVIEW software while electrochemical measurements werecollected via CH Instruments potentiostat (CHI730E).

Electrochemical Characterization:

Electrochemical measurements were performed in a glass cell with asimple 3-electrode configuration and carried out in a 0.1 M pH 7phosphate buffer solution (PBS) bubbled with argon for ˜5 min beforeuse. The 0.1 M PBS was made by combining 4 mL of 1 M NaH₂PO₄ and 6 mL of1 M Na₂HPO₄ and diluting with 90 mL of deionized Milli-Q water. Theelectrochemical measurements used either the FeS₂ coated 3 mm glassycarbon electrode, or a bare 3 mm glassy carbon electrode, or a bare 2 mmPt electrode (CH Instruments) as the working electrode, a Pt wire (CHInstruments) as the counter electrode, and an Ag/AgCl electrode withporous Teflon tip (CH Instruments) as the reference electrode; however,the experiments were reported using the reversible hydrogen electrode asthe reference potential. Linear sweep voltammetry (LSV) experiments wereperformed at 1 mV s⁻¹ with a CH660E potentiostat (CH Instruments). Allreported LSVs were corrected for double-layer capacitance anduncompensated resistance. The FeS₂ discs stability test along with thePt TOF calculations were performed by utilizing chronoamperometry at anapplied potential of −0.14 V vs RHE, with stirring provided by amagnetic stir bar to overcome mass transfer limitations. Time averagedata was recorded with each data point corresponding to an averagecurrent over 5 minutes.

The scanning electrochemical microscopy (SECM) reactivity mappingexperiments were performed in a Teflon cell using either the FeS₂ discscoated 3 mm glassy carbon electrode, or a bare 3 mm glassy carbonelectrode, or a bare 2 mm Pt electrode as the substrate, a 200 μm Ptultramicroelectrode (UME) as the SECM tip, a 200 μm Pt wire (ElectronMicroscopy Instruments) as the counter electrode, and an Ag/AgClelectrode with porous Teflon tip as the reference electrode with 0.1 MPBS, bubbled with argon for ˜10 min, as the electrolyte. The SECM tipelectrode was positioned approximately 100 μm away from the substrateelectrode before scanning. Scanning was performed with the substrateelectrode at a negative potential sufficient to produce hydrogen whilethe SECM tip electrode was held at a positive potential sufficient tocollect hydrogen. A 666.67 μm/s scanning speed was utilized with 100 μmsteps over a 3500 μm×3500 μm area for the Pt substrate electrode, or a4000 μm×4000 μm area for the FeS₂ discs coated glassy carbon and bareglassy carbon substrate electrodes.

The SECM substrate generation/tip collection (SG/TC) experiments wereperformed in a Teflon cell with an FeS₂ discs coated 200 μm Auultramicroelectrode (UME) as the substrate, a 200 μm Pt UME as the SECMtip, a 200 μm Pt wire (Electron Microscopy Instruments) as the counterelectrode, and an Ag/AgCl electrode with porous Teflon tip as thereference electrode with 0.1 M PBS 0.5 mM (dimethylaminomethyl)ferrocene(DMAMFc), bubbled with argon for ˜10 min, as the electrolyte, and scanrate of 10 mV/s.

Results and Discussion.

This Example utilizes a solution hot-injection method to create uniquehyperthin iron sulfide nanostructures with atomic layer thickness. Inthe first step of the synthesis, an octadecylamine (ODA) ligand wasadded to a Fe²⁺ solution, which formed ˜3-5 nm iron nanoparticles asseen in the transmission electron microscopy, TEM, images (FIG. 1A, 1E).The ODA ligand acts as both a reducing agent for the Fe²⁺ (Equation 1)and as a capping layer on the subsequent nanocrystal formation. Next,upon injection of sulfur, the iron seed particles oxidize to form Fe²⁺and S_(x) ²⁻ moieties (Equation 2) and these species form the FeS₂nanostructures via Equations 3 and 4.

$\begin{matrix}{{{Fe}^{2} + {2\; e^{-}}}\overset{L}{arrow}{Fe}^{0}} & {{Equation}\mspace{14mu} 1}\end{matrix}$Fe⁰+S_(x)→Fe²⁺+S_(x) ²⁻  Equation 2

Fe²⁺+S_(x) ²⁻→FeS+S_(x-1)  Equation 3

FeS+S_(x) ²⁻→FeS₂+S_(x-1) ²  Equation 4

It was found that low dimensional structural formation could be tunedthrough adjustments of the initial sulfur concentration with two primarylow dimensional FeS₂ nanostructures being observed. Changing the Fe:Sratio present in the precursor solutions formed either distinct wire ordisc nanostructures. It was determined that a 1:6 Fe:S ratio yieldedwires (FIGS. 1B, 1F) uniformly separated by a tightly packed layer ofligand (FIG. 1D) with a spacing of approximately 2.7 nm. Increasing theFe:S ratio to 1:24 resulted in the formation of discs (FIGS. 1C, 1G)which appear in a stack of thin sheets also separated by a ligand layer.

The kinetics of the wire and disc reactions were tracked through (1)time dependent growth patterns monitored by TEM (FIGS. 2A-2C and 2E-2G);(2) energy-dispersive X-ray spectroscopy (EDS) measurements (FIG. 2I),which monitored the rate at which the Fe:S stoichiometry changed; and(3) UV-Vis-IR spectra (data not shown) which showed the changes in therelative peak heights of the FeS₂ characteristic set of absorbance peakswith respect to reaction time. The transition to FeS₂ was kineticallydifferent between the wire and disc structures. The wire reaction (FIGS.2A-2C) occurred relatively slowly, with the iron seed particles stillpresent for several minutes into the reaction. Examination of the discreaction (FIGS. 2E-2G) revealed faster kinetics with initial discformation occurring within seconds of the injection and the seedparticles being consumed minutes earlier than the wires. The EDSmeasurements correlate well with both the TEM and absorbance data. Thewires reached the desired 1:2 stoichiometry after 30-60 minutes andmaintained that stoichiometry for the duration of the reaction. The 1:24Fe:S precursor ratio (discs) showed a faster conversion. Within 30seconds, disc formations were observed, and 10 minutes into the reactionthe discs reached a 1:2 Fe:S ratio. The scanning electron microscopy(SEM) characterization of the final wire structure (FIG. 2D) showed thebulk wires forming long bundled strands with lengths well over a micronwhich come together to form a porous sponge-like structure held togetherby ligand-ligand interactions. The SEM image of the final disc structure(FIG. 2H) also showed the stacking of discs to form larger structureswith a range of diameters from 300 to 800 nm that most likely areconnected by ligand-ligand interactions.

Raman spectroscopy was used to further study the material (FIG. 2J).Both the wire and disc structures share a characteristic set of Ramanpeaks at 291 and 358 cm⁻¹.^(24, 27) The combination of the EDS data andordered nanostructures within TEM images led to the conclusion thatthese peaks correspond to an ordered FeS₂ structure. However, thethinness of these materials may not provide for accurate determinationof the more typical Raman active modes^(28, 29) of the usual FeS₂ phases(e.g. pyrite, marcasite) nor of the other typical Fe_(1-x)S phases.Similarly, X-ray diffraction (XRD, data not shown) confirmed thepresence of Fe nanoparticles in the early stages but did not determinethe phase in the final products, potentially due to the thinness of thematerial leading to insufficient scattering volume. However, analysiswith high-resolution transmission electron microscopy (HRTEM) andelectron diffraction as described below (see FIGS. 6A-6B) showed thatthe wire and disc structures were polycrystalline with majoritymarcasite phase.

The HER electrocatalytic activity of the nanostructured FeS₂(drop-casted on a glassy carbon electrode) was measured via linear sweepvoltammetry (LSV) at 1 mV/s in 0.1 M pH 7 phosphate buffer solution(PBS). FIG. 3A (solid lines) shows the capacitance and i-R correctedLSVs for the champion FeS₂ 1D wires, 2D discs, 3D cubes (TEM for the 3DFeS₂ cubes shown in FIG. 7) along with a blank glassy carbon electrodeand Pt electrode. By synthesizing 2D FeS₂ nanostructures, the onsetpotential was shifted to very near the thermodynamic potential forhydrogen evolution (0 V vs RHE) indicative of exceptionally highelectrocatalytic activity. In fact, these novel 2D FeS₂ nanostructureshave an overpotential less than 50 mV larger than that of Pt.Triplicates of the LSV experiments obtained from separate batches of theFeS₂ 1D wires, 2D discs, and 3D cubes (the duplicate data is not shown)showed good reproducibility between samples with variances attributed tovariability in electrode fabrication.

To quantify the electrocatalytic activity, the pseudo-steady statemeasurements were fit to the single-electron transfer Butler-Volmerequation (dashed lines, FIG. 3A) assuming no mass-transfer effects(Equation 5, below). This allowed for accurate exchange currentdensities and transfer coefficients to be obtained for each structure.It should be noted that at this pH and scan rate, the mass-transferlimited regime is reached at much lower currents than typically seen atfaster scan rates (FIG. 8) or at lower pH.³⁰ From the Butler-Volmerequation, the exchange current density, which is a measure of kineticsfor the hydrogen evolution reaction,³¹ for the FeS₂ discs, wires, andcubes were determined to be 2.2, 0.32, and 0.41 μA cm⁻², respectively,while Pt had an exchange current density of 8.0 μA cm⁻². This shows thatonly the 2D FeS₂ disc nanostructures had an exchange current density onthe same order of magnitude as Pt. A secondary calculation of theexchange current densities and transfer coefficients were obtained fromTafel analysis (FIG. 3B). The Tafel plot yielded exchange currentdensities of 6.3, 1.7, 0.30, and 0.47 μA cm⁻² for Pt and the FeS₂ discs,wires, and cubes, respectively. All of the exchange current densities ascalculated via the Tafel plot are within 25% of those values calculatedwith the Butler-Volmer equation. Transfer coefficients and Tafel slopesfor each electrode are shown in Table 1, below. Similar transfercoefficients and Tafel slopes between the 2D FeS₂ disc structure and Ptsuggests that the 2D FeS₂ structure has a Pt-like HER mechanism inneutral pH.

TABLE 1 Kinetic parameters obtained from the Tafel analysis and Butler-Volmer Equations for Pt and the FeS₂ Discs, Wires and Cubes. Tafel SlopeButler-Volmer Slope j⁰ j⁰ [mV decade⁻¹] [μA cm⁻²] α [μA cm⁻²] α Discs 761.7 0.76 2.2 0.71 Wires 91 0.30 0.64 0.32 0.65 Cubes 200 0.47 0.29 0.410.31 Pt 71 6.3 0.82 8.0 0.71

To determine the stability of the FeS₂ discs, a constant potential of−0.14 V vs RHE was applied for over 125 hours in 0.1 M pH 7 PBS whilevigorously stirring, and the reduction current was measured as afunction of time. The data (not shown) indicated that the reductioncurrent did not change significantly over the 125-hour experiment. Thissuggests that, by maintaining reducing conditions (i.e. negativepotentials) sufficient to evolve hydrogen, the FeS₂ discs catalyst isstable for generating hydrogen from water under neutral pH conditions.The turnover frequency (TOF) was calculated from Equation 6, below,using the data of reduction current versus time and the electrochemicalsurface area, which was calculated from double-layer capacitancemeasurements. The TOFs of the FeS₂ discs and the Pt electrode weredetermined to be 149 electrons h⁻¹ and 644 electrons h⁻¹ under the sameconditions (FIG. 9) with the FeS₂ discs having more than double theelectrochemical surface area than the Pt electrode.

To verify that hydrogen was evolving from the surface of the 2D FeS₂discs, an HER electrochemical reactivity map was obtained via scanningelectrochemical microscopy (SECM, FIGS. 4A-4D). SECM is a powerfultechnique for imaging the reactivity of electrocatalytic surfaces andfor studying electrochemical reactions.³²⁻³⁴ FIG. 4A shows the schematicfor obtaining a hydrogen evolution electrochemical reactivity map. Herethe catalytic electrode was held at a negative potential sufficient toevolve hydrogen and a 200 μm Pt SECM tip electrode was held at apositive potential sufficient to oxidize any hydrogen present insolution. The SECM tip electrode was placed c.a. 100 μm above thecatalyst electrode and was scanned across the catalyst surface while thetip current was recorded as a function of tip position. Areas wherehydrogen is being generated by the catalyst electrode will produce anoxidation current on the SECM tip electrode at that position.

HER electrochemical reactivity maps were obtained on the Pt electrode,the 2D FeS₂ discs coated on glassy carbon, and a bare glassy carbonelectrode (FIGS. 4B-4D). Both the Pt (FIG. 4C) and the 2D FeS₂discs-coated electrode (FIG. 4D) showed oxidation currents on the SECMtip electrode, indicative of hydrogen existing in solution over eachelectrode. For comparison, FIG. 4B showed no hydrogen in solution forthe bare glassy carbon electrode operated at the same potential at whichFIG. 4C was generated. Thus, via these SECM electrochemical reactivitymaps, it can be concluded that the FeS₂ discs catalyst is indeedgenerating hydrogen gas.

Substrate generation/tip collection (SG/TC) SECM was used to estimatethe Faradaic efficiency for hydrogen generation (FIGS. 11A-11D). In thisexperiment a 200 m Pt tip electrode was positioned over a 200 μm Ausubstrate electrode coated with FeS₂ discs, and linear sweep voltammetrywas performed on the FeS₂ electrode and the Pt tip electrode collectedthe evolved hydrogen as a function of potential. Using the SG/TC SECMtechnique, the faradaic efficiency of the FeS₂ discs for hydrogenevolution is estimated to be 92±8%.

Additional Results and Discussion

Additional results and discussion relating to materials characterizationand electrochemical characterization are provided.

Materials Characterization:

The low temperature phase of the FeS₂ 1D and 2D structures could not becharacterized by Raman spectroscopy or XRD measurements because of theatomic layer thickness of the structures. However, HRTEM and electrondiffraction measurements described below (see FIGS. 6A-6B) confirmed thestructures were polycrystalline with majority marcasite phase. As such,the standard characterization methods (Raman/XRD) of these 1D and 2Dstructures yielded results that are different than the 3D hightemperature phases. UV-Vis-IR measurements were taken of aliquots ofwire and disc reactions at 0.5, 5, 10, 30, 90, and 240 minutes.Initially peaks are not readily apparent, but after reaction timesgreater than 1 minute the characteristic peaks appear at 320, 430, 530,660, and 700 nm in both the wire and the disc reactions (data notshown). (See Wilcoxon, J. P.; Newcomer, P. P.; Samara, G. A. Solid StateCommun. 1996, 98, 581-585 and Gong, M.; Kirkeminde, A.; Ren, S. Sci.Rep. 2013, 3, 2092.) Optically the 2-D growth of the discs causes rapidpeak formation with little difference between scans at earlier and laterreaction times. The 1D growth of the wires shows slower peak formation.In the early stages of the wire reaction, the shorter wavelengthsdominate the spectra and have the highest relative intensities. However,as the reaction progresses, the peaks at 660 and 700 nm proceed tobecome the dominant peaks with the 700 nm peak appearing as a lowerintensity shoulder. The absorbance spectra stabilize when the 700 nmshoulder eclipses the 660 nm peak achieving the highest maximum relativeintensity of all the characteristic peaks.

For clarity, the growth of the 700 nm absorbance peak over the course ofthe reaction was plotted (FIG. 5) and shows a strong correlation withthe EDS data monitoring stoichiometry. The fact that the 700 nm peakgrows in and does not appear to shift as it grows suggests a structural(i.e. stoichiometry) change instead of a size dependent shift in peakabsorbance because of confinement. Additionally the rate and time atwhich the 700 nm peak reaches its maximum could be used as an in-situmethod for characterizing wires or discs formation as well as reactionprogress, respectively.

Characterization by XRD (data not shown) was used to determine crystalstructure and structural evolution during the course of the reaction.Initially, it was found that crystalline iron particles had formed priorto the injection of the sulfur, and persisted for the first few minutesof the reaction. The match to iron oxide is a result of exposure ofthese small particles to open air during the measurement. Further alongin the reaction, the iron oxide peaks disappear and what is left ismostly noise with small peaks observed that do not match with thetypical 1:2 Fe:S structures of iron pyrite or marcasite. Numerous otherFe_(x)S_(y) XRD patterns were investigated but none matchedappropriately. This may be caused by the limitations of the XRD whenscanning particles with atomic layer dimensions. In addition, as thelow-dimensional FeS₂ hyperthin nanostructured materials are susceptibleto the irradiation damage (primarily the oxidation issue) during the XRDmeasurement, it did not serve as a reliable source to characterize theFeS₂. However, the HRTEM and electron diffraction measurements describedbelow (see FIGS. 6A-6B) confirm the majority marcasite phase of thematerials.

TEM images of crystalline FeS₂ discs are shown in FIGS. 6A-6B. In FIG.6A, a low magnification image shows a FeS₂ disc with 1 μm diameter.Select area electron diffraction (SAED) pattern was taken from the redcircled area to confirm its crystallinity, as shown in the insert. Highresolution TEM image of a FeS₂ disc is shown in FIG. 6B. FFT of thesquared area (insert) shows characteristic diffraction peaks of the(020), (110), and (200) planes from marcasite FeS₂, indicating the imagewas taken from its [001] direction. Thus, this data shows that thematerial is polycrystalline with majority marcasite phase.

Electrochemical Characterization:

Equation 5 is an approximate form of the single-electron transferButler-Volmer Equation (Bard, A.; Faulkner, L., Electrochemical Methods:Fundamentals and Applications. John Wiley & Sons, Inc: 2001, New York,pp. 100) assuming an irreversible reaction with no mass transfer effectsand a large overpotential. The equation describes the relation betweenthe current density, j, and the overpotential, 77, which is equivalentto the electrode potential versus RHE. The exchange current density, j₀,and the transfer coefficient, α, are the two kinetic parameters thatwere regressed from the LSV data, F is the Faraday constant, R is theuniversal gas constant, and T is temperature.

$\begin{matrix}{j = {j_{0}e^{- \frac{\alpha \; \eta \; F}{RT}}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

As stated above, triplicates of the LSV experiments were obtained fromseparate batches of the FeS₂ 1D wires, 2D discs, and 3D cubes (data notshown). While the champion data from each set has been reported above,good reproducibility was obtained with no overlap between the worst 2Ddiscs and best 1D wires, and no overlap between the worst 1D wires andbest 3D cubes. Differences between samples were attributed tovariability in electrode fabrication. For comparison, FIG. 7 shows theTEM image of the 3D FeS₂ cubes to the 1D wire and 2D disc structures.

As described above, it was also demonstrated that high stability ofthese catalysts can be obtained when held under reducing conditions toevolve hydrogen. However, exposure of the FeS₂ nanostructures tooxidizing potentials causes deactivation. Thus, it should be noted thatthe catalytic activity of the FeS₂ nanostructures decreases betweenscans when multiple cyclic voltammetry experiments are performed in therange of +0.3 to −0.3 V vs RHE on the same electrode.

For all electrochemical measurements, uncompensated resistancemeasurements were made via the “iR Comp” function on the CHI 660EPotentiostat software. The potential of all LSVs were then corrected forthe uncompensated resistance. Measurements of uncompensated resistancein the samples were in the range of 100-520Ω. Double-layer capacitancewas also corrected for by subtracting background current (or currentdensity) obtained from an extrapolated CV performed in the potentialregion before the onset of hydrogen evolution. The raw experimental datawith no capacitance or iR correction for the data shown in FIG. 3A,above, and for the duplicate LSV experiments is not shown.

As stated above, linear sweep voltammograms (LSVs) of the FeS₂nanoparticles were carried out at 1 mV/s so that a pseudo-steady statecurrent could be reached at each potential. In FIG. 8A, it can be seenthat the system becomes mass transfer limited at modest currentdensities, but both the Pt and the FeS₂ disc reach the mass transferlimited regime at similar current densities. The mass transfer limitedregime for Pt at 50 mV/s (FIG. 8B) occurs at much higher currentdensities than for Pt at 1 mV/s because of the transients and steeperconcentration gradients occurring from a faster scan rate. FIG. 8C showsthe LSVs with current density normalized by scan rate to illustrate thedependence of current on scan rate.

The turnover frequency (TOF) was calculated with Equation 6.

$\begin{matrix}{{TOF} = \frac{{iN}_{Avo}}{A_{EC}{FN}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

where i is the current, N_(Avo) is Avogadro's constant, A_(EC) is theelectrochemical surface area, F is Faraday's constant, and N isapproximated as 10¹⁵ atoms cm⁻². The electrochemical surface area wasdetermined from standard active surface area calculations based oncapacitance assuming 20 μF cm⁻² as described above. The surface area was4.8 and 1.0 cm² for the FeS₂ discs and Pt, respectively. The roughnessfactor (electrochemical surface area/geometric area) was found to be 68for the FeS₂ discs and 31 for Pt. The raw data from which the current,i, was obtained can be seen in FIG. 9.

Similar measurements of the electrochemical surface area were alsoobtained for the wires and cubes. The wires had an electrochemicalsurface area of 2.3 cm² and the cubes had an electrochemical surfacearea of 0.56 cm². This corresponds to roughness factors of 33 and 8respectively. FIG. 10 shows the LSVs showing the surface area normalizedcurrent as a function of potential.

The Faradaic efficiency of hydrogen evolution on the FeS₂ disc wasestimated from a SECM substrate generation/tip collection (SG/TC)experiment (data not shown). In this experiment a 200 μm Pt tipelectrode was positioned over a 200 μm Au electrode coated with FeS₂disc substrate electrode. Linear sweep voltammetry was performed on theFeS₂ electrode and the Pt tip electrode collected the evolved hydrogenas a function of potential. However, the first step in performing SG/TCSECM to obtain the faradaic efficiency for hydrogen evolution is todetermine the maximum collection efficiency using an outer-sphere redoxmediator. The maximum collection efficiency can be under 100% because ofmisalignment of the tip to the substrate.

The redox mediator used to determine the theoretical maximum collectionefficiency was (dimethylaminomethyl)ferrocene, DMAMFc, (E^(1/2)˜0.35 Vvs. Ag/AgCl). However, the FeS₂ discs were found to deactivate underoxidizing potentials required to approach, align the tip and substrate,and perform the DMAMFc⁺ SG/TC SECM. Thus, a quick approach and map wererequired, resulting in misalignment of the substrate and the SECM tip.This resulted in a maximum collection efficiency for DMAMFc/DMAMFc⁺, of1.6%. The DMAMFc/DMAMFc⁺ SG/TC SECM data was corrected for thecollection efficiency. Also the large tip/substrate distance caused adelay in the collection of DMAMFc⁺, which was corrected for.

The hydrogen SG/TC data was corrected for the maximum collectionefficiency of the DMAMFc SG/TC experiment. In addition the largetip/substrate distance caused a delay in hydrogen collection by the SECMtip due to a large diffusion distance between the tip and substrate, aswas also the case for the DMAMFc⁺ collection. Corrections forcapacitance were also performed as described above. Due to the lowcollection efficiency is was difficult to exactly quantify the faradaicefficiency of hydrogen because small variations in the collectionefficiency of DMAMFc propagate through in calculating the Faradaicefficiency of hydrogen. However using this method an estimate ofhydrogen Faradaic efficiency of the FeS₂ discs was determined to be92±8%.

CONCLUSION

In summary, this Example reports a novel synthesis method to create 2DFeS₂ nanostructures, which significantly improves the electrocatalyticperformance of earth-abundant FeS₂ for the HER. It was found that themorphology and stoichiometry of the FeS₂ could be tuned by the initialsulfur concentration. The 1D FeS₂ wires and the 2D FeS₂ discs showedhigher electrocatalytic activity compared to the conventional 3D FeS₂cubes for the HER under neutral pH conditions. In fact, the 2D FeS₂materials displayed excellent electrochemical activity—similar toplatinum—with high exchange current densities and an onset potential forhydrogen evolution near the thermodynamic potential. The 2D FeS₂ discsalso proved to be remarkably stable demonstrating the ability togenerate hydrogen for over 125 hours when under reducing conditions.Using SECM, it was verified that hydrogen was being generated from boththe Pt and FeS₂ discs electrodes, but not from a bare glassy carbonelectrode.

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The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. An electrode comprising a FeS₂ electrocatalyticmaterial, the FeS₂ electrocatalytic material comprising FeS₂nanostructures in the form of FeS₂ wires, FeS₂ discs, or both, whereinthe FeS₂ wires and the FeS₂ discs are hyperthin having a thickness inthe range of from about the thickness of a monolayer of FeS₂ moleculesto about 20 nm, and further wherein the FeS₂ nanostructures arepolycrystalline comprising a non-pyrite majority crystalline phase. 2.The electrode of claim 1, wherein the thickness is in the range of fromabout the thickness of a monolayer of FeS₂ molecules to about 10 nm. 3.The electrode of claim 1, wherein substantially all the FeS₂ discs haveat least partially curved edges.
 4. The electrode of claim 3, whereinsubstantially all the FeS₂ discs have entirely curved edges.
 5. Theelectrode of claim 3, wherein the FeS₂ discs comprise circular FeS₂discs, elliptical FeS₂ discs, or both.
 6. The electrode of claim 1,wherein the non-pyrite majority crystalline phase is marcasite.
 7. Theelectrode of claim 1, wherein the FeS₂ wires are assembled in the formof bundles, wherein neighboring wires within each bundle aresubstantially aligned along their lengths and separated by a ligandlayer and the FeS₂ discs assembled in the form of stacks, whereinneighboring discs within each stack are substantially aligned alongtheir planes and separated by a ligand layer.
 8. The electrode of claim7, wherein the bundles or stacks are randomly oriented with respect toone another to define a plurality of pores distributed through thematerial.
 9. An electrode comprising a FeS₂ electrocatalytic material,the FeS₂ electrocatalytic material comprising FeS₂ nanostructures in theform of FeS₂ discs, wherein the FeS₂ discs are hyperthin having athickness in the range of from about the thickness of a monolayer ofFeS₂ molecules to about 20 nm, and further wherein substantially all theFeS₂ discs have at least partially curved edges.
 10. The electrode ofclaim 9, wherein substantially all the FeS₂ discs have entirely curvededges.
 11. An electrochemical system for catalyzing an electrochemicalreaction, the system comprising: (a) an electrochemical cell configuredto contain a fluid comprising an electrochemical reactant; (b) theelectrode of claim 1 in contact with the fluid; and (c) a counterelectrode in electrical communication with the electrode of claim
 1. 12.The electrochemical system of claim 11, further comprising a powersource configured to apply an electrical potential across the electrodeof claim 1 and the counter electrode in order to generate free electronsfor inducing a reduction reaction at the electrode of claim
 1. 13. Theelectrochemical system of claim 12, wherein electrochemical reaction isthe hydrogen evolution reaction, the fluid is an aqueous electrolytesolution, and the reduction reaction is the generation of H₂ fromhydrogen ions and the free electrons.
 14. A method for making theelectrode of claim 1, the method comprising: (a) injecting a firstprecursor solution comprising sulfur (S), the first precursor solutionhaving a first temperature, into a second precursor comprising iron(Fe), the second precursor solution having a second temperature, to forma reaction mixture, and (b) allowing the reaction mixture to react at areaction temperature for a reaction time, wherein a ratio of Fe:S in thefirst and second precursor solutions is selected to achieve thenanostructures in the form of FeS₂ wires, FeS₂ discs, or both, whereinthe FeS₂ wires and the FeS₂ discs are hyperthin having the thickness inthe range of from about the thickness of a monolayer of FeS₂ moleculesto about 20 nm, and further wherein the FeS₂ wires and the FeS₂ discsare polycrystalline comprising the non-pyrite majority crystallinephase.
 15. The method of claim 14, wherein substantially all thenanostructures are in the form of FeS₂ discs and the ratio of Fe:S inthe first and second precursor solutions is in the range of from about1:24 to about 1:38.
 16. A method of using the electrode of claim 1 tocatalyze an electrochemical reaction, the method comprising exposing theFeS₂ electrocatalytic material to a fluid comprising an electrochemicalreactant under conditions sufficient to induce the reduction of theelectrochemical reactant at the FeS₂ electrocatalytic material-fluidinterface to form a reduction product or under conditions sufficient toinduce the oxidation of the electrochemical reactant at the FeS₂electrocatalytic material-fluid interface to form an oxidation product.17. The method of claim 16, wherein the electrochemical reaction is thehydrogen evolution reaction, the fluid is an aqueous electrolytesolution, the electrochemical reactant comprises hydrogen ions which areinduced to form hydrogen gas as the reduction product in the presence offree electrons.
 18. The method of claim 17, wherein the electrochemicalreaction occurs at about neutral pH.
 19. The method of claim 17, whereinsubstantially all the nanostructures are in the form of FeS₂ discs. 20.The method of claim 19, wherein the method is characterized by anoverpotential of no more than about 90 mV as determined at 0.1 mA/cm² ina 0.1 M phosphate buffer at a pH of 7 and a scan rate of 1 mV/s.